Interpretive Summary: Variable-rate air-assisted sprayers that match spray outputs to target can reduce pesticide use and minimize off-target loss. Droplet size is one of the most important factors that influence spray quality on target areas. To elucidate the mechanistic principles underlying air-assisted five-port nozzles in variable-rate applications, the magnitude of influence of spray parameters on droplet size distributions must be determined. Consequently, the effect of the modulation rate, spray solution, air velocity and liquid pressure on droplet size distributions produced from an air-assisted variable-rate nozzle was determined. The optimal operating conditions to minimize droplet size variations were to maintain a constant liquid pressure and use modulation rates of 20-100%. Droplet sizes also varied with the air velocity; however, this variation was acceptable for use in the air-assisted nozzle. This information will be useful for future development of new sprayers to achieve a reliable spray technology synchronizing spray outputs with canopy structures in real-time.

Technical Abstract:
Verification of droplet size distributions is essential for the development of real-time variable-rate sprayers that synchronize spray outputs with canopy structures. Droplet sizes from a custom-designed, air-assisted, five-port nozzle coupled with a pulse-width-modulated (PWM) solenoid valve were measured under various operating conditions. Parameters included five air velocities (0, 10, 30, 50, 70 m/s), three spray solutions (water-only, a water-surfactant solution, and a water-drift retardant solution), five operating pressures (138, 207, 276, 345, 414 kPa), and seven pulse-width-modulation rates (10, 20, 40, 50, 60, 80, 100%). Droplet sizes did not vary significantly with modulation rates of 20 to 100%. The variation of droplet size was highly significant with spray solution formulation and operation pressure and was slightly significant with air velocity. A dimensionless parameter which was the ratio of liquid viscosity and relative discharge velocity to surface tension was correlated with DV0.1, DV0.5 and DV0.9 of droplets for the three spray solutions. Droplet size variations were minimized when the variable-rate nozzle was operated at a constant liquid pressure and when a 10% or lower modulation rate was not used. The variation of droplet sizes due to changes in the air velocity for water-only and surfactant amended sprays was acceptable for the new sprayer development.